In electronic circuits, voltage sources and current sources are well known electronic building blocks. Voltage-controlled current sources include voltage-to-current converters. Voltage-to-current converters transform an input voltage signal into an output current signal. The current signal is relatively independent of a load impedance. Linearized voltage-to-current converters generate a current signal that is substantially linearly proportional to a voltage signal. Linearized voltage-to-current converters are used, for instance, in convolvers, correlators, adaptive filters, multipliers, dividers, squaring circuits, integrators, analog-to-digital converters, phase-locked loops, and other applications in which a current signal, rather than a voltage signal, is required as in input signal to another circuit.
Many schemes exist for implementing linearized voltage-to-current converters. For example, in an integrated circuit, a voltage-to-current converter can be provided using a metal-oxide semiconductor (MOS) transistor coupled to a reference resistor. For instance, an N-channel MOS (NMOS) transistor with a source coupled to a reference resistor can convert a gate voltage into a linearly related drain current. The NMOS transistor and reference resistor can be fabricated with 0.9 micrometer technology using a P-type semiconductor substrate, with the diffused N-type semiconductor region (or tub) for the source being sufficiently doped as to also provide an N-type reference resistor. Although this approach is compact, it has certain drawbacks. First, the resistance of the N-type reference resistor may be difficult to control due to processing variations in doping levels. Furthermore, the resistance of the N-type reference resistor may drift during operation in response to voltage or temperature changes. Consequently, the value of an N-type reference resistor may vary by as much as 20 percent. Similar drawbacks attend to P-type reference resistors.
Reference resistors with well-controlled resistances are available. For instance, an external resistor may be interconnected to the integrated circuit, but the interconnection may require an additional bonding pad on the surface of the integrated circuit. Or, reference resistors integral to the integrated circuit can be rendered more accurate using laser trimming or link blowing, but these techniques require additional processing and testing steps.
Operational amplifiers may also be used as voltage-to-current converters by inputting a voltage signal to the noninverting input and having the output drive a Darlington transistor pair which supplies the desired output current. A drawback of operational amplifiers is the need for relatively complex circuitry and a significant amount of silicon area for its implementation.
Based on the foregoing, there is a need for an accurate, linearized, and cost-effective voltage-to-current converter for implementation in an integrated circuit.